As an automobile catalyst for purifying an exhaust gas, a 3-way catalyst is employed which carries out the oxidation of CO and HC and the reduction of NO.sub.x simultaneously to purify an exhaust gas. With regard to such a catalyst, a catalyst has been known widely in which a porous loading layer comprising .gamma.-alumina is formed on a heat-resistant support substrate, such as cordierite, and a noble metal, such as Pt, Pd and Rh, is loaded on this loading layer.
By the way, the purifying performance of such a catalyst for purifying an exhaust gas depends greatly on the air-fuel ratio (A/F) of an engine. For example, when the air-fuel ratio is large, namely on a lean side where the fuel concentration is lean, the oxygen amount in the exhaust gas increases so that the oxidation reactions of purifying CO and HC are active, on the other hand, the reduction reactions of purrying NO.sub.x are inactive. Conversely, for example, when the air-fuel ratio is small, namely on a rich side where the fuel concentration is high, the oxygen amount in the exhaust gas decreases so that the oxidation reactions are inactive and the reduction reactions are active.
Whilst, in automobile driving, in the case of urban driving, the acceleration and deceleration are carried out frequently so that the air-fuel ratio varies frequently within the range of from adjacent to the stoichiometric point (ideal air-fuel ratio) to the rich state. In order to cope with the low fuel consumption requirement in such driving, a lean-burn control is needed in which a mixture containing oxygen as excessive as possible is supplied. However, the oxygen amount is large in an exhaust gas emitted from a lean-burn engine, and accordingly the reduction reactions of purifying NO.sub.x are inactive. Therefore, it is desired to develop a catalyst for purifying an exhaust gas which can fully purify NO.sub.x in the exhaust gas which is emitted from the lean-burn engine and contains oxygen in a large amount.
For this reason, a catalyst for purifying an exhaust gas has been proposed conventionally in which a zeolite, such as mordenite, having an HC adsorbing ability is employed as a catalyst loading layer (for example, Japanese Unexamined Patent Publication (KOKAI) No. 4-118,030). In this catalyst for purifying an exhaust gas, HC are adsorbed while the temperature of the exhaust gas is low, the adsorbed HC are released by increasing the temperature of the exhaust gas, thus the NO.sub.x in the exhaust gas are reduced, and accordingly the NO.sub.x conversion can be improved.
Moreover, since the zeolite has many acidic sites and are acidic, it is good in terms of the HC adsorbing ability and adsorbs HC in the exhaust gas. Therefore, even in an exhaust gas of oxygen-rich atmosphere, the atmosphere adjacent to the catalyst is from the stoichiometric point to a rich atmosphere where the HC are present in large amounts, the NO.sub.x are reacted with the adsorbed HC by the catalytic action of a loaded noble metal, and thereby they are reduced and purified.
A method of purifying an exhaust gas is proposed in which a specific catalyst is provided and liquefied HC whose number of carbon atoms is 5 or more are added on an upstream side with respect to it (Japanese Unexamined Patent Publication (KOKAI) No. 6-165,918). In this method, while the downstream-side catalyst is in a low activated state, HC are supplied from the upstream side, the NO.sub.x are reduced by these HC, and the improvement of the NO.sub.x conversion is intended. Here, since the HC whose number of carbon atoms is 5 or more, namely the high-grade HC, are gradually subjected to cracking by heat in the exhaust gas, when the downstream-side catalyst is put into an activated state at a temperature of from 300 to 500.degree. C., they are turned into low-grade HC whose number of carbon atoms is less than 5 by cracking, and thus it is considered that the NO.sub.x can be reduced securely.
Therefore, the present inventors considered as follows. In a method of purifying an exhaust gas in which mordenite having an HC adsorbing ability, namely, a porous oxide being a zeolite in a wide sense, is employed as a support, and in which HC are supplied, and in a catalyst having the arrangement, the 3-way purifying performance can be further improved.
However, the following are revealed. The low-grade HC are better than the high-grade HC in terms of the NO.sub.x reducing force as aforementioned, but they were inferior thereto in terms of the adsorbing force to the noble metal in the catalyst. Namely, the low-grade HC are less likely to adsorb onto the noble metal in the activated downstream-side catalyst.
Therefore, in the case where the HC are supplied from the upstream side, even if they are high-grade, they are eventually subjected to cracking by heat on the downstream side by 10 cm from the upstream-side of the catalyst, and they are turned into low-grade HC, CO or CO.sub.2, since the low-grade HC are less likely to adsorb onto the noble metal in the activated downstream-side catalyst, they do not react with the NO.sub.x and are likely to be emitted as they are. Hence, as a result, it is less likely to acquire the expected improvement of the NO.sub.x conversion.
Therefore, it is an object of the present invention to provide a method of purifying an exhaust gas and a catalyst for purifying an exhaust gas which can exhibit a high NO.sub.x conversion even in an exhaust gas in an oxygen-rich atmosphere.
By the way, when a catalyst exhibits a purifying ability, a temperature of 300.degree. C. or more is needed in general. However, immediately after driving is started, since the temperature of an exhaust gas flowing in the catalyst is low, the inherent purifying ability of the catalyst is not exhibited, and accordingly there arises a problem in that the HC and NO.sub.x are emitted as they are substantially.
Hence, in Japanese Patent Application No. 6-524,106, an international patent application, a purifying apparatus is disclosed in which a 3-way catalyst or an oxidizing catalyst is disposed on an upstream side of an exhaust gas flow, subsequently an NO.sub.x removing catalyst is disposed, and furthermore a 3-way catalyst or an oxidizing catalyst is disposed on a downstream side.
According to this purifying apparatus, since the exhaust gas, which is heated by the reaction heat in the upstream-side 3-way catalyst or oxidizing catalyst and whose temperature is increased, is flowed into the NO.sub.x removing catalyst, it is possible to reduce and remove the NO.sub.x even in such a case as immediately after driving is started where the exhaust gas temperature is low.
In Japanese Unexamined Patent Publication (KOKAI) No. 2-135,126, there is disclosed an exhaust gas purifying apparatus in which an adsorbing catalyst adapted for adsorbing HC and having a zeolite and a noble metal is disposed on an upstream side of an exhaust gas flow, and a 3-way catalyst is disposed on a downstream side.
According to this exhaust gas purifying apparatus, the HC are adsorbed on the adsorbing catalyst when the exhaust gas temperature is low, the HC are removed from the adsorbing catalyst as the temperature of the flowing-in exhaust gas increases. Then, the HC flowed to the downstream side reduce the NO.sub.x on the downstream-side 3-way catalyst. Accordingly, it is possible to reduce the emission of the HC even in such a case as immediately after driving is started where the exhaust gas temperature is low, and the NO.sub.x purifying performance is improved.
Moreover, in Japanese Unexamined Patent Publication (KOKAI) No. 7-174,017, there is disclosed a catalytic apparatus in which a 3-way catalyst is disposed on an upstream side of an exhaust gas flow, and an adsorbing catalyst adapted for adsorbing HC and having a zeolite and a noble metal is disposed on a downstream side.
According to this catalytic apparatus, the HC are adsorbed on the adsorbing catalyst when the exhaust gas temperature is low, the HC are removed from the adsorbing catalyst as the temperature of the exhaust gas flowing into the adsorbing catalyst increases, but the HC are oxidized and removed on the adsorbing catalyst when the temperature is the activation temperature of the noble metal or more. Accordingly, it is possible to reduce the emission of the HC even in such a case as immediately after driving is started where the exhaust gas temperature is low.
However, in the purifying apparatus disclosed in Japanese Patent Application No. 6-524,106, there are drawbacks in that it takes long for the temperature of the exhaust gas flowing into the NO.sub.x removing catalyst to increase to the activation temperature or more of the NO.sub.x removing catalyst, and in that the purifying performances of the HC and NO.sub.x are not sufficient when the exhaust gas temperature is low.
In the purifying apparatus disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 7-174,017, the upstream-side 3-way catalyst is cooled by the exhaust gas, for example, at the time of deceleration, but there arises a case where the purification of the HC is made difficult by cooling the downstream-side adsorbing catalyst as well.
In addition, in the aforementioned catalysts, the NO.sub.x purifying performance is not satisfactory when the exhaust gas is such an exhaust gas as emitted from a diesel engine and containing SOF (Soluble Organic Fraction) in a large amount in the HC. The reason for this is described as follows.
Among the HC adsorbed on the adsorbing material, the relatively high molecular-weight HC, such as the SOF, have a property that they are less likely to be eliminated at a low-temperature range. Accordingly, in the low-temperature range, the HC are less likely to flow to the downstream side, the efficiency of the reduction reactions of the NO.sub.x is low when the exhaust gas containing SOF in a large amount is purified. When the SOF adsorbs onto the zeolite, since the SOF is less likely to be eliminated, the active sites of the noble metal are covered therewith so that there occurs a poisoning in which oxygen-deficiency state arises. When such a poisoning occurs, the activity of the noble metal is lost so that the purifying ability decreases.
Therefore, it is an object of the present invention to provide a method of purifying an exhaust gas and a catalyst for purifying an exhaust gas which can furthermore efficiently remove the HC and NO.sub.x even in such a case as immediately after driving is started or at the deceleration where the exhaust gas temperature is low.
Moreover, it is another object of the present invention to provide a method of purifying an exhaust gas and a catalyst for purifying an exhaust gas which can furthermore efficiently remove the NO.sub.x even in an exhaust gas containing SOF in a large amount.
A. zeolite exhibits a cracking action, and a zeolite, such as mordenite, ZSM-5 and super-stable type Y zeolite (US-Y), exhibits an especially high cracking action. Accordingly, by employing these zeolites as a catalyst support, the SOF in the diesel exhaust gas is subjected to cracking, and it is turned into low-molecular weight HC which are likely to react, and thereby it is possible to furthermore efficiently reduce and purify the NO.sub.x.
A zeolite is tectoaluminosilicate chemically, and zeolites have been known which have a variety of Si/Al ratios. It has been revealed that, depending on this Si/Al ratio, the catalytic properties of a zeolite vary greatly.
Since a zeolite of a small Si/Al ratio has many acidic sites, and exhibits a high cracking ability and a high HC adsorbing ability, it is good in terms of the NO.sub.x purifying ability. However, in a zeolite having a small Si/Al ratio and many acidic sites, the HC adsorbed in the pores are carbonized to be easily subjected to caulking, and enclose within the pores, as a result, there arises a drawback in that the HC adsorbing ability decreases chronologically.
Moreover, in a zeolite having a small Si/Al ratio and many acidic sites, the acidic sites are easily lost by the elimination of Al (tetra-coordination in the zeolite structure is turned into hexa-coordination) when it is subjected to a hydrothermal durability treatment, and there arises a drawback in that the cracking ability decreases. In addition, in a catalyst in which a noble metal is loaded on such a zeolite, the noble metal is grown granularly by the elimination of Al resulting from the hydrothermal durability treatment, and there arises a drawback in that the activity decreases.
On the other hand, a zeolite having a large Si/Al ratio has a small amount of acidic sites, and accordingly exhibits a low cracking ability. However, since no caulking occurs, the chronological decrement of the HC adsorbing ability does not occur, and since the granular growth of the noble metal resulting from the elimination of Al is suppressed, there arises an advantage in that it is good in terms of the durability.
Therefore, it is an object of the present invention to provide a method of purifying an exhaust gas and a catalyst for purifying an exhaust gas which can furthermore efficiently remove the NO.sub.x by employing a zeolite having a large Si/Al ratio to suppress the decrement of the HC adsorbing ability and by securing a high cracking ability equivalent to that of a zeolite having a small Si/Al ratio.